/*
 * Copyright 2021-2022 by Nedim Sabic Sabic
 * https://www.fibratus.io
 * All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package entropy

import "math"

// Shannon measures the Shannon entropy of a string.
func Shannon(value string) int {
	frq := make(map[rune]float64)

	//get frequency of characters
	for _, i := range value {
		frq[i]++
	}

	var sum float64

	for _, v := range frq {
		f := v / float64(len(value))
		sum += f * math.Log2(f)
	}

	return int(math.Ceil(sum*-1)) * len(value)
}
